This invention relates to static monitoring systems for use with an electronic manufacturing work station or in a work area. Many present day electronic assembly procedures are sensitive to electrostatic charges, and a typical work area includes a work bench or table having a grounded surface and a wrist strap or other electrical conductor for connecting the operator to ground. In a typical installation, both the working surface and the operator's wrist strap are connected to earth ground such as may be achieved by direct electrical connection to a cold water pipe. Typically the operator wrist strap is a flexible conductor such as the commonly used expansion metallic wrist watch strap.
A variety of configurations have been proposed for achieving the desired control of static charge. However, problems of sensitivity, false alarms, fail safe operation, and the like have been encountered, and it is an object of the present invention to provide a new and improved static monitoring system which provides the desired indications and control while being substantially unaffected by electrical noise and sufficiently sensitive to indicate approaching charges prior to potential damage to work in progress, and at the same time being substantially free of false alarms which unnecessarily inhibit production, and having fail safe characteristics so that the operator can determine if the system is not functioning properly.
It is an object of the present invention to provide for the detection of potentially hazardous static fields within the proximity of the work area by continuously monitoring the work environment for charges entering the work area. Typically this would be the approach of a fellow worker or supervisor. The charges approaching the work station are detected and an alarm is provided well before the charge gets near static sensitive components at the work station.
Another object of the invention is to provide a system which insures that operators and supervisors are grounded, by continuously monitoring the status of the wrist strap. The system will display a safe indication such as a green light when the wrist strap is being worn and is functioning properly, while displaying a fail or red indication and providing an audible alarm when the desired conditions are not met. Separate wrist strap monitors are provided for the operator and a supervisor, with each strap monitored separately and simultaneously.
It is an object of the invention to provide circuitry for indicating that the connections to earth or cold water ground and to circuit or electrical ground are present. Both earth ground and electrical ground are continuously monitored and compared, with an alarm being provided when the difference exceeds a predetermined value.